<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    Array.prototype.groupBy = function(fn) {
    const res = {}
    for(const item of this){
        const key = fn(item);
        if(!res.hasOwnProperty(key)) res[key] = [];
        res[key].push(item)
    }
    return res
};
array = [
  {"id":"1"},
  {"id":"1"},
  {"id":"2"}
], 
fn = function (item) { 
  return item.id; 
}
console.log(array.groupBy(fn)); 
  </script>
</body>
</html>